The new SR 520 Floating Bridge will be the longest floating bridge in the world. Building a bridge that sits on the water comes with several highly unusual construction challenges as well as a few advantages.

Types of pontoons

The new SR 520 floating bridge will be supported by three types of concrete pontoons:

Longitudinal pontoons (21)

These are the largest pontoons at approximately 360 feet long. They form the backbone of the bridge and support the roadway superstructure.

Constructed in Aberdeen facility

Weight: 11,100 tons

Cross pontoons (2)

These mark the ends of the floating bridge section and the transition to the East and West Approach structures.

Constructed in Aberdeen facility

Weight: 10,100 - 10,550 tons

Supplemental stability pontoons (54)

These smaller pontoons help stabilize and support the weight of the new floating bridge.

Constructed in Aberdeen facility (10) and Tacoma facility (44)

Weight: 2,500 - 2,820 tons

Where will WSDOT connect the new floating bridge?

As shown below, crews will install the new floating bridge north of the existing bridge.

How does the floating bridge connect to the stationary bridge?

The moveable, floating bridge connects to the stationary, elevated bridge segment at the east approach with a transition span. The transition span is made up of girders, each 190 feet long and 45 tons. On either end of the span, hinges allow the transition span to move up to 24 inches up and down or side to side to accommodate varying lake water levels.

Click image for an aerial view of how the transition span fits into the new SR 520 (pdf 2.52 mb).

What type of anchors will be used to secure the new floating bridge?

Three different types of anchors will secure the new bridge from shifting during wind and wave action. These types are: fluke anchors, gravity anchors, and drilled shaft anchors.